That presumes that people decide to stop thinking.
A person using a calculator still has to understand precisely what he wants the calculator to do. The calculator is like a tractor to a farmer. The farmer still has to understand exactly what has to be done—what seeds to plant, what fertilizer to use, how much water is required.
The tool removes tedium. It provides leverage.
When a person uses AI to produce code, precise instructions are still required and the code has to be debugged and tested carefully. That's leverage, not the removal of critical thinking.
BTW, people who use computers for mathematical modeling, or engineering, or accounting, or statistics are not mentally lazier than people who don't use computers. They are always significantly more intellectually robust. Computers requires more rigor, not less.
The brain is dumb and as untrained as a muscle that never got exercised, until the brain is trained to use its neural networks to perform tasks that are put to it. Not tasked to do certain things the brain is hard pressed to address a mental task put to it and the person says, particularly in math and logic - “I don’t know”. I am not better at math because my brain is better at math. I am better at math because a few great teachers and I imposed more and more difficult mathematical tasks for the brain to do. In the analogy of a well exercised muscle the brain got stronger.
If people no longer know how to judge an A.I. result, as simple as with math, A.I. becomes more than a “tool”, it is a brain for its human robot it feeds instructions to.